I have been using my M2 for about a year now pretty successfully.
I just saw that Simplify 3D came up with a new enhancement which allows you to print a few layers of soluble PVA between supports and part.

I am now seriously considering installing a second extruder.

How difficult is it to install, set the profiles, and get it running efficiently?

Since I made the conversion to a dual extruder, I can tell you that the installation is pretty straightforward, apart from the somewhat unwieldy cabling (more extruders means more wires crammed into the box, and a mess of wiring packed behind the extruder motors). Getting it running efficiently is not difficult.

It is every bit as precise as the single extruder. I noticed no difference in the quality of my single-extrusion prints or the performance of the machine in building them.

However, using the dual extruders for dual prints has always been a bit of a pain, which is partly a consequence of software limitations and partly a consequence of having two extruders right next to each other without the ability to prevent either one of them dripping while the other one is active. I have not yet had time to play around with Simplify3D v4.0.0, so I can't say how much improvement has been made to some of the worst problems (the worst one I know of was an incorrect handling of the initial extrusion, leading to the second head being retracted when starting, so that several layers of the second filament get missed at the start, compounded by the inability to prime the second head properly by using it to build part of the brim).

As far as the dripping problem, in my experience good dual prints are pretty much limited to PLA plastic (and choose your brand wisely), because everything else drips too much to be useful. Ooze shields and prime pillars work well when they work, but then the size and orientation of a print relative to the distance between the extruder heads mean that sometimes the 2nd extruder is inside the ooze shield, dripping all over the print.

A thin layer of PVA printed at high density is a definite improvement for support structures. I tried PVA some time ago for support and was taken aback by the amount that I needed to use, the wastefulness and expense of it, and the huge amount of time it added to the print to build a solid support structure from ground up. So that feature of Simplify3D is worth looking into. However, it doesn't solve the problem of having soluble PVA drip onto the print.

I think a lot of people are now waiting for reports on the M3 independent dual extruder, which seems like more the right way to do dual extrusion. Having two independent heads on separate motor drives lets you get the inactive extruder out of the way and into a parked position fast, before it has time to drip. Once you have that, all you need is (at most) a prime pillar to get the filament going again. It should not even require an ooze shield. I think the M3 dual coupled with much-needed critical fixes in Simplify3D should make the perfect dual extruder system.

Everything Tim said, and more. I actually had prints fail due to the oozing. I was trying to use an expensive filament for the part and cheap PLA for the support. The PLA nozzle would run along the part as the other nozzle extruded, leaving just enough PLA to destroy the layer adhesion. The independent extruders will solve this problem.
For other parts and filaments, though, the dual is a great device!